Fluid light modulating mediums for image projection apparatus



United States Patent a corporation of New York FLUID LIGHT MODULATING MEDIUMS FOR IMAGE PROJECTION APPARATUS 16 Claims, 4 Drawing Figs.

U.S. Cl l78/7.5

Int. Cl H04n 3/16, H04n 5/44 Field ofSearch l78/S.4,

[56] References Cited UNITED STATES PATENTS 3,274,565 9/1966 Wright l78/7.5D 3,288,927 11/1966 Plump l78/7.5D

Primary Examiner-Robert L. Griffin Assistant Examiner-Albert .l. Mayer Attorneys-Richard R. Brainard, Paul A. Frank, Charles T.

Watts, Leo l. Ma Lossi, Frank L. Neuhauser, Melvin M. Goldenberg and Oscar B. Waddell ABSTRACT: A light modulating fluid of considerably improved properties is prepared by adding a concentration of polymeric material to conventional light modulating fluid used in the projection of self-erasing, rapid decay images. The polymeric material must not only be soluble in the base (conventional) fluid at the image forming temperature but must be soluble therein to the extent that a molecular weight/concentration relationship of the polymeric material can be established in the base fluid, which is productive o2 viscoelastic behavior in the modified fluid. A simple test fol the routine identification of viscoelastic capability in thc modified fluid is described.

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. k .c 3 a H n e8 e n/ FIELD- FLUID LIGHT MQDULATINGMEDIUMS FOR IMAGE PR OJEC'TIO'NAPPARATUS This is a continuation-impart of U.S. Pat. application Ser. No. 560,912, filed May 31, 1966 now abandoned, in the names of Carlyle S. Herrick and Frederick F. Holub and assigned to the assignee of this application.

This invention relates to improvements in fluid light modulating mediums for the projection of self-erasing, rapid decay images in apparatus of the kind wherein a fluid light modulating medium is deformed into diffraction and/or refraction gratings by the impression of electron charges thereon as a function of electrical signals corresponding to the images. One importantaspect of this invention is the discovery that the improved fluid mediums disclosed herein may be erased by the combined effects of self-erasure by the surface tension of the fluid medium and erasure by the imposition of an outside force.

Apparatus employing such a fluid light-modulating medium is described, for-example, in U.S. Pat. No. 2,943,147, Glenn,

Jr. wherein is disclosed a projection system comprising an evacuated glass envelope containing an electron gun for producing an electron beam, which is deflected in a rectangular raster over the surface of a light-transmitting electrondeformable light-modulating medium contained within a portion of the transparent glass envelope. The electron beam is they produce deformations in this surface with the amplitude of the deformationsbeing a function (among other parameters) of the number of-electrons deposited by the electron beam at various points over the surface of the raster area. As a result, the amplitudes of these deformations are a function of the electron beam modulation. Repetition rates of more than one image per second are employed and this is possible because of the rapid decay of each image such-that the same icone fluids, methyl silicone fluids with phenyl silicone additives, methylphenyl silicones containing an average of 2 is set forth in U.S. Pat. No. 3,125,635, Murrayet al. Other.

suitable light modulating mediums are described in the following patent applications assignedtothe assignee of this inventiohzapplication Ser. No. 335,151, Plump, filed Jan. 2, 19.64

(now U.S. Pat. No 3,288,927); application Ser. No. 392,107, Perlowski, .lr., filed Aug. 26, 1964 (now U.S. Pat. No. 3,317,664); application Ser. No. 392,110, Perlowski, Jr., filed Aug. 26, 1964 (now U.S. Pat. No. 3,317,665); and application Ser. No. 422,169, Plump, filed Dec. 30, 1964 (now abancloned). Among the preferred fluids described in the latter applications are polybenzyltoluene (PET) and polybenzylnaphthalene PBN Differential charge deposited by the electron beam produces the aforementioned deformation in the light modulating medium. The'intensity of light refracted or diffracted by the deformation rises. exponentially to a maximum and thereafter decays as the charge on the surface of the light modulating medium decays due to charge transport through the bulk of the light modulating medium to the underlying conducting layer. The time it takes for the deformation to reach 63 percent ofmaximum value in response to a step force function has been described in application Ser. No. 419,475 True et al. (filed Dec. 18, 1964 now U.S. Pat. No. 3,385,925 assigned to the assignee of this invention) as the mechanical time constant, and the time it takes for the electric force producing the deformation to decay to 63 percent of its peak value has also been described therein as the electrical time constant. For purposes ofexplanation herein these terms will I be referred toas the rise time" and "decay time, respectively. For the successful operation of systems relying solely on self-erasure, it is important that the sum of the rise and decay area of fluid is employed for the formation of aquick succession ofimages. v

in the instant invention the, light modulating medium is a thin layer of light transmissive, fluid in which the electron beam forms phase diffraction gratings (and/or refraction gratings) in the form of alternate hills and valleys caused by the deforming effect of theelectron beam. The adjacent -valleys are spaced apart by a predetermined distance such that each portion of light incident on a respective small area or point of the medium is deviated in a direction orthogonal to the direction of the valleys. The intensity of the deviated light is a function of the depth of the valley and diminishes as. au-. toerasure of the deformation occurs wherein thehills and valleys diminish and the fluid is ready for the writing of a new image.

Conventional deformable light modulating mediums are described in the aforesaid U.S. Pat. No. 2,943,147. In general,

(between about 253C. and 150C.) of from approximately 100- to about 50,000 centistokes (c.s.), and are Newtonian in behavior. Further, the mediumv must be compatible with the underlying conducting coating. Criteria are alsostated with respect to the volume resistivity of the deformable medium as varying within the range of from about .lO'l to about 104 ohm-cm, with the average resistivity at the stable (quiet mode been shown that when-electron charges are deposited on thick of operation) thickness being approximately 101 .ohm-cm.

However, the volume resistivity of .fluidsemployed in apparatuses different from that described in U.S. Pat. No.-

2,943,147 may be lower than 1011 ohm-cm. and such fluids,.as

well, will be improved by the application of this invention thereto.

Various materials are listed in U.S. Pat. No. 2,943,147 as suitable light modulating mediums, for example, methyl siltimes be of the order of the duration-oft! field ofscan, i.e., the

deformation should be reduced to. about one third of its peak value by the time the electronbeam is in a position to deposit another pattern of charge at that point.-

For the aforementioned unmodified fluids it has been found that the rise time to deform the fluid-is a function principally of the viscosity of the light modulating fluid, the depth of the light modulating fluid layer, the grating line spacing, and the surface tension of the fluid. -As higher viscosity fluids are selected for use, larger rise times result an vice versa As thinner layers of fluid are employed the rise time is increased and vice versa. If the grating line spacing (distance from crest to crest of adjacent hills) is increased, therise time is increased and vice versa. The decay'time, which'is an expression of the autodecay feature, is principally a function of the manner ofconductionof charges through the fluid layer. The decay time varies in a direct relationship with the product of viscosity and depth, and in an inverse relationship with electronbeam current. Ithas also been foundin the aforementioned unmodified fluids that mobility of charge carriers involved in thedecay of. charge in the fluid varies in an inverse relation with the viscosity.

It has been found that relatively thick layers of light modulating fluid are necessary to achieve uniformly good deformation or writingcharacteristics. Unfortunately, however, ithas layers of fluid to produce desired deformation therein, additional unwanted deformations bearing no relationship to the desired deformations are created. These latter deformations,

hereinafter referred to as "noise, are appreciable in depth in relation to the desired deformations and are substantial in extent. As a result, they produce deviation of light which deleteriously affects the contrasts in the projected image and these deviations become a part of the projected image destroying the distinctness of image boundaries. These unwanted deformations can vary in their effect from causing a haziness to actually obliterating all traces of the signal modulation. For a particular light modulating fluid, if the thickness of the fluid in the raster area is reduced below a certain critical value, these unwanted deformations do not occur. This critical value is referred to as the critical quieting thickness" (COT). Also, it has been found that CQT values vary in inverse relationship to the current of the electron beam, i.e. for smaller electron beam currents, the COT has a larger value.

Such noise has not been a problem in thermoplastic recording (described in US Pat. No. 3,1 13,179 Glenn, Jr.), because after its deformation the .deformed medium carried by the recording tape is moved away from the electron beam and is changed to the solid state before noise has had time to form therein. As described hereinabove, the fluids to which the instant invention apply provide rapidly decaying images, which automatically erase themselves (or are erased by the combined effects of the surface tension of the fluid medium and an imposed force) and the same portion of fluid receives a new image as the fluid remains in the fluid state. In the latter case the writing is ofa rapidly repetitive nature while in the former case the writing is nonrepetitive.

The depth of fluid at which the COT occurs becomes manifest in the following manner. When the thickness of the fluid is less than the COT, each fluid has its own capacity to conduct electric charges impressed on its surface through the fluid to the ground plane base (conductive layer). It is believed that this charge transfer occurs both by electrical con duction phenomena and by various flow patterns coordinated with the raster lines. If the thickness of the fluid is greater than the COT, flow patterns uncoordinated with the raster lines or with the modulated signals are generated in the fluid. These uncoordinated flow patterns deform the surface resulting in the deviation of light not resulting from signal modulation. This phenomenon is known as optical noise" or "noise. For a given value of thickness of the fluid layer, this uncoordinated flow occurs as the current density of the electron beam is increased from zero to some value beyond which a sudden and widespread initiation of uncoordinated flow occurs in the fluid. The fluid thickness at this depth is at the COT for this value of current.

Operation with a fluid thickness of less than the COT value is desirably, because the noise referred to hereinabove is destructive of the signal-modulation produced image. More effective operation, greater flexibility for the system and improved image production are achieved by increasing this value of the COT whereby operation with a thicker layer of fluid without noise is made possible.

It is therefore a prime object of this invention to provide a light modulating fluid of considerably improved COT proper ties whereby optical noise therein caused by uncoordinated flow patterns is eliminated.

It is also an object of this invention to obtain the additional advantages to be gained from increased COT in light modulating fluids whereby the optical efficiency of the projection system will be increased and the rise time of the fluid will be decreased to bring it into better operational balance with the decay time.

It is another object of this invention to provide an improved light modulating fluid yielding the capacity for greater light efficiency for the projection, system or, alternatively, for a reduction in the damage to the fluid writing medium.

These objects and other advantages are obtained, therefore, by adding to and dissolving in a conventional light modulating fluid used in the projection of self-erasing, rapid decay images, a concentration of less than about 35percent by weight of the base fluid of a higher molecular weight polymeric material, which addition creates viscoelastic properties in the fluid and preferably also contributes shear thinning sensitivity to the fluid. The prime concern is to convert a fluid light modulating medium, which is Newtonian in behavior, to a solution exhibiting certain non-Newtonian behavior.

Other objects and features of the invention will become apparent to those skilled in the art as the disclosure is made in the following description including the annexed drawing in which:

FIG. I is a schematic representation of optical and electrical elements comprising a projection system wherein fluid light modulating compositions in accordance with this invention may be employed.

FIG. 2 illustrates the testing procedure for determining the incidence of viscoelastic behavior in solution of fluid light modulating mediums prepared in accordance with this invention, and

FIGS. 3A and 3B represent the comparative monochromatic light outputs for one frame (two fields) from a given diffraction grating (series of lines of uniform spacing) wherein interlacing lines are employed when unmodified fluid (FIG. 3A) and the viscoelastic fluids of this invention (FIG. 3B) are used.

As is indicated in FIG. 1, the projection system comprises an evacuated glass envelope 10 containing an electron gun I] mounted therein for generating electron beam 13, which is deflected in the shape ofa rectangular raster over the surface of a transparent deformable light modulating liquid 15 that is disposed over at least a portion 17 of a conducting surface 19 disposed within envelope 10 in some convenient manner. Electron beam 13 is modulated by a television signal applied to deflection means (not shown) in electron gun 11. Region 21 ofdeformable medium 15 is coincident with the raster area covered by the beam 13 and deformations are imposed thereon by electrons from beam 13 that are attracted to conducting coating 19 as described hereinabove.

These deformations are utilized to diffract light from a light source 23 in an optical system which is illustrated as including a lens 24 that images light source 23 on the surface of medium 15 through a bar and slit system 25. Another lens 29 images the slits of system 25 on the bars of another bar and a slit system 31 in the absence of deformations in the surface of the deformable medium. However, any deformations phase diffract (or refract) the light so that it passes through the slits in the system 31 with an intensity that corresponds to the amplitudes of the deformations and thusthe amplitudes ofthe applied television signal. The light passing through system 29 is imaged by a projection lens 33 on a screen 35 after reflection from a mirror 37.

No attempt has been made to disclose the more sophisticated details of the apparatus, which would be used in commerce, however, the details given herein suffice to properly evaluate those aspects offluid light modulating mediums to which this invention is directed and in which the benefits of this invention have been successfully demonstrated. In order to more definitely describe these benefits the following definitions are introduced:

The term viscoelastic behavior is defined in the text, Elastic Liquids A. S. Lodge, on page 71. Thus, a liquid is viscoelastic if (a) the stress in the flowing liquid does not become instantaneously isotropic (or zero) as soon as the liquid is confined to a constant shape or (b) the flowing liquid does not remain at constant shape as soon as the stress is made instantaneously isotropic (or zero).

"Shear thinning sensitivity" is defined as the property of a liquid whereby the viscosity of the liquid decreases as the shear rate is increased, and vice versa.

In practice, testing for the initiation of the desired viscoelasticity in a liquid is easily accomplished by dipping a pointed probe into the liquid and then withdrawing it. If the liquid is not viscoelastic no connection remains between the end of the probe and the surface of the liquid. If the liquid exhibits the desired'viscoelasticity a thread (a filament having a length-to-diameter ratio of about to l, or greater) will remain connecting the probe and the surface of the liquid as the probe is moved away therefrom as shown in FIG. 2 wherein thread 41 has been drawn from liquid 42 in container 43 by probe 44. Commonly such threads can be drawn to lengths ofover one-halfinch.

, Asa practical consideration, the above-described thread test can be conducted on the modified fluid at the operating temperature of the parent fluid. This temperature is somewhat lower than the operating temperature of the modified fluid. Repeating the test at the new, higher operating temperature confirms the viscoelastic behavior of the modified fluid under operating conditions.

By the use of this test suitable polymer additives (those soluble in the light modulating fluid and exhibiting satisfactory radiation stability) can be checked to determine the threshold molecular weight required for the specific concentration tested. Any polymer soluble in the light modulating fluid will at high enough molecular weight induce viscoelastic behavior in the fluid due to chain entanglement. Therefore, having selected some given molecular weight of polymer additive, it is simply necessary to increase the concentration of polymer additive in the fluid until the development of threads in the conduct of the test described above. If the viscoelastic behavior is not manifest at any concentration, then a higher molecular weight ofthe polymer additive must be tested.

Having determined the threshold molecular weight for a given concentration of a particular polymer additive dissolved in a particular light modulating fluid, it is then possible to determine the value of a K factor by substitution in the formula:

Threshold Molecular Weight:

Knowing the value of the K factor for the particular combination of fluid and polymer additive, one may estimate the threshold molecular weight at any desired concentration. This estimate is then verified by the same test procedure, i.e. formation of threads.

By way of example for the PET polystyrene combination, K has a value of about 2500.

Various low molecular weight preparations approximating polystyrene formulations disclosed as materials for thermoplastic layers in U.S. Pat. No. 3,113,179, Glenn, Jr. were tested to determine the suitability of these preparations as fluid mediums for the projection of self-erasing, rapid decay images. Each of the liquids exhibited too high viscosity and for this and other reasons reported below were found unsuitable. The requisite operating viscosity should be about 1,000 c.s., while the value of viscosity closest to 1,000 c.s. obtainable with the low molecular weight polystyrene preparations was 4,000 c.s.

Thus, a mixture of 70 percent by weight polystyrene (average molecular weight 30,000), 25 percent by weight mterphenyl and 5 percent by weight ofa copolymer (85 percent styrene, percent butadiene) was tested at 4,000 c.s. No thread was formed with the probe test and the material volatilizes at operating temperature. A mixture of 70 percent by weight polystyrene (average molecular weight 30,000) and 30 percent by weight m-terphenyl was tested at 4,000 0.5. with results similar to those reported in the previous example. A sample of 100 percent polystyrene (average molecular weight 20,000) was heated to 260C. to reduce the viscosity thereof to 4,000 c.s. during testing. Visible decomposition took place and, when care was taken not to accumulate a ball of fluid at v the end ofthe probe, no thread could be drawn from the sample.

A proper way to compare modified and unmodified fluids is to measure CQT under conditions of equal surface deformation created by equal electron beam currents and resulting in equal quantities for the sums (for each fluid) of the rise time and the decay time. Spacing of the surface deformations should be near the minimum required in use. This was the procedure used in the examples reported herein. In particular the summation of times is measured as the fraction of a field time (one-sixtieth second for television) required for the light diffracted and/or refracted by the surface deformations generated by 14 me. modulation (spacing in a 22 X26 mm. raster) to decrease to the fraction of Us of maximum light intensity. In measuring an unknown fluid the summation of times is first established at about 0.7 field by changing temperature until the desired value is obtained. This fixes the liquid temperature and establishes the operating viscosity after which CQT is measured as the maximum liquid layer thickness at which the liquid surface is not perturbed by the aforementioned uncoordinated flow patterns (noise).

Certain experimental behavior has been noted and forms some basis for theoretical explanation of the beneficial effects of the addition of high molecular weight polymers to writing fluids. These observations are set forth herein, however, without intent of being limited to any particular theoretical explanation.

When the chain entangling polymer is dissolved in the writing fluid the mobility of the electric charges moving through the fluid from the deformed surface to the conducting underlayer (charge carrier mobility) remains about the same, but the viscosity increases substantially. Since the writing fluids are compared at equal summations of the rise and decay times (which in effect means at almost constant viscosity) the operating temperature of the solution (additive-modified writing fluid) is also increased to offset the increase in viscosity. Such increase in temperature increases charge carrier mobility. This appears to be a general phenomenon with the use of these additive materials. In all instances in which viscoelastic behavior has been observed in the prepared solution, a broad range of molecular weights (and thereby of concentrations below about 35 percent by weight of the parent fluid) of polymer additive may be found with which an increase of CQT will be effected. When a polymer additive ofmolecular weight and concentration within the proper range is employed, the prepared solution not only exhibits viscoelastic behavior but also displays'increased charge carriermobility as described above.

Mention has been made hereinabove of the preferability of employing a polymer additive, which renders the parent fluid shear thinning sensitive as well as viscoelastic at the operating temperature for the modified fluid. ln modified fluids exhibiting shear thinning sensitivity it has been found that the rise time is appreciably diminished in comparison to the unmodified parent fluidjThis behavior is of particular advantage in that it enables more effective operation by increasing the light efficiency and, in the case of color projection, provides improved color image quality.

As noted above, the operating temperature must be raised to a higher value after the addition of a high molecular weight polymer. The extent to which the operating temperature is raised depends upon whether erasure is to be accomplished solely by self-erasure or by combined self-erasure and imposed erasure. Because of this necessity of raising the operating temperature, in producing a practical writing fluid the vapor pressure characteristic of the writing fluid after modifications must be kept low enough as not to interfere with the electron beam generating and controlling mechanisms by diminishing the vacuum in the housing. 7

In addition to exhibiting viscoelastic behavior, being soluble in the writing fluid, and exhibiting radiation resistance in order to minimize radiation damage to the composite fluid, the polymer additive preferably should be a glassy-type polymer having a low glass temperature and with respect to such breakdown of the polymer additive as may occur, the resulting byproducts should not be incompatible with the performance requirements of the basic writing fluid.

If a crystalline type high molecular weight polymer additive be employed, it should have the quality of not forming crystals in the fluid during use.

One of the most promising aspects of the projection of images in apparatus employing the fluid light modulating mediums discussed herein is the projection of color images. Such a system is described the aforementioned Pat. application Ser. No. 419,475, True et al. As is described therein, one of the important qualities required in the light modulating mediums for projecting color images in addition to its writing sensitivity is the extent of green/red dynamic compromise which is available therewith.

Tests have established that the green/red dynamic compromise is always improved by the addition of polymer additives in the manner described herein and in any event such addition will not diminish the writing sensitivity of the parent fluid. Further, there appear to be no significant parameters deleteriously affected by the addition of this material. Other additives (nonpolymeric) have been employed. A few of these nonpolymer additives have exhibited an increase in COT, however most of these have markedly decreased the writing sensitivity of the basic fluid. On the other hand some nonpolymer additives appear to increase the writing sensitivity although they have had no effect on the COT. Tests with high molecular weight additives have shown that the beneficial effects of using multiple doping materials are usually cumulative. Therefore, it is contemplated that in addition to using high molecular weight polymer additives other additive materials may be employed in combination therewith to produce optimum performance.

The minimum amount of high molecular weight polymer additive required to produce the requisite increase in COT is dependent on the particular combination of polymer additive and basic fluid, acting as the solvent. As indicated hereinabove, a threshold molecular weight exists for each polymer and solvent combination above which individual polymer chains of the additive material interact with each other or entangle each other to produce viscoelastic properties in the solution.

Among the suitable additives are the high molecular weight linear methylphenylpolysiloxanes disclosed in the aforementioned Patnode patent having at least 20 mol percent siliconbonded phenyl groups, high molecular weight polystyrene and polyphenylene ethers, for example, those materials'described in British Pat. No. 930,993. By reference, these patents are made part of the disclosure of the instant application.

As maybe seen from the following examples the amount of additive material having the desired effect can be very small. In these examples the parent and modified fluids were'exposed to an electron beam in a housing (such as shown in FIG. 1)

reproducing operating conditions, i.e..those conditions at i which the proper summation of rise time and decay time is tures indicated herein are the temperatures of the fluid. during the particular operation and the amounts of additive material is expressed as percent by weight of base fluid.

EXAMPLE I A parent PBT fluid having a COT of about 9 microns at 0.7 field for the summation of the time constants at 30C. and 4 microamperes/in. writing current was modified by dissolving in the parent fluid 2 /i'percent by weight of highly branched chain polystyrene having a molecular weight of 2.4 X10". The temperature was raised to 495C. at which point the summation of rise time and decay time was again 0.7 field at 4.0 microampereslin. CQT of the solution was then found to be about l8 microns and viscoelasticity was exhibited at this operating temperature by the conduct of the thread test.

EXAMPLE2 A base PBT fluid having a COT of about 5 microns at 0.7 field (32C.) and 4.0 microamperes/in. was modified by dissolving 2%percent by weight therein of the same highly 4 requisite viscoelasticity by test.

. sxaurnea A parent?! fluid having a COT of about 4%r'nicrons at 4.0 microamperes/inf and 0.7' field (36C.);was modified by dis solving in it Zlkpercent by weight of linear chain siloxane consisting essentially of recurring units of the formula polystyrene having a molecular weight of l X10. For the combination of ingredients at 0.7 field (42.5'C.) and 4.0 microamperes/in. the COT was about 15 microns and the modified fluid was viscoelastic.

EXAMPLE 4 EX A MPLE 5 PBT base fluid having a COT of about 5 microns at 0.7 field (32C.) and 4.0 microamperes/in. was modified by dissolving therein 16 percent by weight of linear chain polystyrene having a molecular weight of 9.7 X10. At 0.7 field (57C.) and 4.0 microamperes/in. the COT of the solution was 15 microns and the solution was found to be viscoelastic.

EXAMPLE 6 PBT base fluid having a COT of about 5 microns at 0.7 field (32C.) and 4.0 microamperes/in. was modified by dissolving therein 32 percent by weight of linear chain polystyrene having a molecular weight of 9.7 X10. At 0.7 field (92C.) and 4.0 microamperes/in. the COT of the solution was measured and found to be 16 microns. Under these operating conditions .the modified fluid exhibited the desired viscoelasticity upon being tested.

EXAMPLE 7 A parent PBT fluid having a COT of about 9 microns at 0.7 field (30C.) and 4 microamperes/i'n. was modified by dissolving therein l6 percent by weight of linear polystyrene having a molecular weight of 2 X10. At 0.7 field (53C.) and 4 microamperes/in.*8 microns.

viscoelastic qualities were not observed at the operating temperature and the charge carrier mobility was not significantly altered.

EXAMPLE 8 Base PBN fluid having a COT of 6 microns at 4 microamperes/in. and 0.7 field (50C.) was modified by dissolving in it 2 percent by weight of linear chain polystyrene having a molecular weight of l X10. At 0.65 field and 4 microamperes/in. at a temperature of 62C. The CQT was 18 microns and the modified fluid, when tested, exhibited the proper viscoelasticity.

EXAMPLE 9 A parent methylphenylslloxane fluid,

CH; CH; CH: mc-de-o-iu-oi-cm aHs till all having a COT of 7 microns at 0.7 field at 28C. an-d4 microamperes/i'n. was modified by dissolving in the parent fluid '6 percent by weight of a linear polymethyl phenyland having a molecular weight of about 4 X10 and an intrinsic viscosity of 26.4. At 0.7 field (42C.) and 4.0 microamper eslini, the COT was l5 microns and at this operating temperature testing established the required viscoelasticity.

EXAMPLE- 10.

The base methylphenylsiloxane fluid of example 9 was modified by dissolving in it lpercent by weight of linear methylphenylsiloxane polymer of the general formula set forth in example 9 having an. intrinsic viscosity of 11.9 and a molecular weight of about X10. At 0.65 field (40C.) and 4 microamperes/in. the COT of the solution was 16 microns and viscoelastic, as shown by test.

EXAMPLE I] To the base fluid of example 9was added Zpercent by weight of a material prepared by copolymerizing 4 parts diphenyl tetrasiloxane and 1 part methylethyl tetrasiloxane to an intrinsic viscosity of 6.68. At 0.7 field (38C.) and 4 microamperes/inFlLS microns and testing verified that the modified fluid had the requisite viscoelastici- EXAMPLE 12 Methylphenylsiloxanefbase fluid as described in example 9 was modified by dissolving therein 3.8 percent by weight of linear methylphenylsiloxane polymer as described by the general formulain example 9 and having an intrinsic viscosity of 1.28. At 0.7 field (32C.) and 4 microamperes/in.", the COT was 8 microns showing marginal improvement and exhibited viscoelasticity bythe thread test.

EXAMPLE 13 EXAMPLE 14'.

PBT base fluid having a CQT'of about microns at 0.7 field (28C.) was modified; by dissolving therein 1 percent by weight of poly-(2-phenyl-6-methyl-l ,4-phenylene ether), having an intrinsic viscosity of 1.03. At 0.7 field (40 C) and 4 microamperes/in. l2 microns and exhibited requisite viscoelasticity upon testing. The

preparation of this poly-(2-phenyl-6-methyl-l,4-phenylene The unique adaptability of writing-fluids modified in accordance with this invention to impose erasure has been illustrated in an apparatus in which interlacing lines were written by the electron beam in the second half of each frame. Each interlace line, when written compressesout the peaks of the previous set of lines into new valleys and inverts the valleys of the previous set of lines into new peaks. This application of force in essence erases substantially all of the previous profile impressed in the writing fluid replacing it with a new profile of information. This capacity of the modified fluid for accepting imposed erasure, therefore, permits operation with substantial increase (as much as 100 percent) in light efficiency over operation in which the erasure is solely passive (self-erasure). In essence, as will be explained in connection with FIG. 3B, this amounts to an increase in the throughput of light per field inthe dark field system. I In those-operations or equipment in which the additional light efficiency is not required, the writing beamcurrent density may be proportionately decreased thereby diminishing the damage to the writing medium increasing the life of the fluid and of the electron gun, as well.

For example, in an interlaced system ordinarily operating at 2 niicroamperes/inF, the light efficiency was increased by about 100 percent. In this particular system the increased light was not necessary and this increased capacity was utilized by operating with decreased writing beam current density, i.e. l microamperes/in.-

FIG. 3A is a graphic representation of the light output (a narrow range of adjacent light frequencies) from a given diffraction grating wherein interlacing lines are employed and the writing fluid is the unmodified conventional fluid. The total lumen output fora single field is proportional to the area under curve x. The graph in FIG. 313 represents operation under the same conditions, e.g. writingbeam current, illumination, diffraction grating, etc., except that the writing fluid is modified in accordance with this invention and has a substantially higher viscosity thereby necessitating a higher operating temperature. As is readily evident, the lumen outether) is described in the aforementioned British Pat. No.

Other suitable additive polymers for PET and PBN include, for example, polyvinylnaphthalene, polyacenapthalene and poly-2-vinlybenzofuran. Likewise, other additives for methylphenyl'siloxane base fluids and silicate fluids include various copolymers of diphenyl tetrasiloxane and dimethyl tetrasiloxane.

Although the improved light modulating fluid described herein has been prepared by modifying a conventional light modulating fluid to illustrate this invention, the criteria necessary for any fluid (single fluid; mixture or solution) to operate satisfactorily as a light modulating medium are identified 1 herein and such fluids may be selected by reference thereto. Thus, it is contemplated that a fluid, i.e. asingle fluid, need only exhibit, viscoelastc behavior evidenced as shear thinning sensitivity and/or the aforementioned thread formation to be put/field is considerably increased. With the unmodified fluid (FIG. 3A) the profile impressed on the fluid has .been substantially erased by the surface tension'of the fluid by the time the interlacing lines are imposed on the system at the end of the first field. In the operation illustrated in FIG. 38 using the higher viscosity modified fluid the extent of self-erasure (curve y) is much less. The operating temperature could, of course, be increased sufficiently such at that self-erasuremay be fully relied upon, however, it has been found that the operating temperature can be lowered and reliance be placed on imposed erasure. Thus, under the conditions illustrated, by

the time the interlacing lines are imposed (point a) by the electron gun, these lines are able to, and do, almost instantaneously execute the greatest amount of the requisite erasure (curve w) and thereafter impose the new profile (curve z) for repetition of the cycle.

Thus, another important aspect of this invention is the discovery of the inherent capacity of the viscoelastic fluids of this invention to respond to imposed erasure in the manner illustrated at a higher operating viscosity for the writing fluid than has previously -beenfound to be acceptable. Although the outside force imposing erasure in the illustration of this phenomenon set forth above comes from interlacing, it is possible that other force applications can be employed to exercise the same unique mechanism.

We claim:

I. In a fluid light modulating medium foruse in apparatus in which a thin layer of said fluid light modulating medium is supported on a conducting plane located relative to means for producing an electron beam so that said beam is directed at said plane to build up charge in said fluid light modulating medium, which charge produces self-erasing deformation in the surface of said layer, each such deformation acting to diffract light directed at said layer from a light source in a light 1 optical system, the diffracted light being projected by the optical system as a function of each deformation to form self-erasing, rapid,decay images, the improvement comprising said fluid having in solution therein polymeric material, said polymeric material being soluble in said fluid at the image said probe, the formation of such a filament being indicative of the requisite viscoelasticity.

2. The improvement recited in claim 1 wherein the polymeric material is polystyrene.

3. The improvement recited in claim 1 wherein the polymeric material is a methylphenylsiloxane' polymer.

. 4. The improvement recited in claim 1 wherein the.

polymeric material is poly-(2-phenyl-6-methyl-l,4-phenylene ether).

5. A system for the projection of self-erasing, rapid-decay images, said system comprising a container having a quantity of fluid light modulating medium, a conducting surface and an electron gun enclosed therein, said conducting surface being adapted to receive a coating of said fluid light modulating medium over atleast a portion thereof and being located within said container in position to receive an electron beam from said electron gun impinging on said portion, a light optical system being disposed relative to said container adapted to direct light through said container, through said portion and the fluid light modulating medium supported thereon and into projection apparatus, said fluid light modulating medium having additive material dissolved therein, said additive material comprising a polymeric material soluble in said fluid light modulating medium at the image forming temperature thereof and being present therein in anamount ranging from' an amount effective to produce viscoelastic behavior in said fluid light modulating medium up to about 35 percent by weight of the additive-free fluid, the requisite property of viscoelasticity being ascertainable by dipping a pointed probe into said fluid light modulating medium at the image forming temperature thereof, withdrawing said probe therefrom and inspecting for the formation of a filament having a length-to-diameter ratio of at least about 100 to 1 interconnecting the free surface of said fluid light modulating medium and said probe, the formation of such a filament being indicative of the requisite viscoelasticity.

6. The projection system recited in claim 5 wherein the polymer is polystyrene.

7. The improvement recited in claim 1 wherein the solution comprises polybenzyltoluene containing polystyrene dissolved therein.

8. The improvement recited in claim 1 wherein the solution comprises polybenzylnaphthalene containing polystyrene dissolved therein.

9. The projection system recited in claim 5 wherein the polymer is poly-(Z-phenyl-fi-mcthyll ,4-phenylene ether).

10. The projection system recited in claim 5 wherein the fluid light modulating medium comprises polybenz yltolu'ene containing poly-(Z-phenyl-ti-methyl-l ,4-phenylene ether) dissolved therein.

ll. The projection system recited in claim 5 wherein the polymer is a methylphenylsiloxane polymer.

12. The projection system recited in claim 5 wherein the fluid light modulating medium comprises methylphenylsiloxane containing a methylphenylsiloxane polymer dissolved therein.

13. The projection system recited in claim 5 wherein the fluid light modulating medium comprises silicate liquid containing a methyjphenylsiloxane polymer dissolved therein.

14. A metho for increasing the critical quieting thickness of a fluid light modulating medium for use in apparatus in which a thin layer of said fluid light modulating medium is supported on a conducting plane located relative to means for producing an electron beam so that said beam is directed at said 'plane to build up charge in said fluid light modulating medium, which charge produces self-erasing deformation in the surface of said layer, each such deformation acting to diffract light directed at said layer from a light source in a light optical system, the diffracted light being projected by the optical system as a function of each deformation to form self-erasing, rapid-decay images comprising the step of dissolving in said fluid light modulating medium an amount of polymeric material ranging from an amount effective to produce viscoelastic behavior in said fluid light modulating medium at the image forming temperature up to about 35 percent by weight of said fluid light modulating medium, the requisite property of viscoelasticity being ascertainable by dipping a pointed probe into the solution at the image forming temperature thereof, withdrawing said probe therefrom and inspecting for the formation of a filament having a length-to-diameter ratio of at least about 100 to l interconnecting the free surface of said solution and said probe, the formation of such a filament being indicative of the requisite viscoelasticity.

15. In the method of preparing a system for the projection of images in which a thin layer of a fluid light modulating medium is supported on a conducting plane located relative to means for producing an electron beam so that said beam is directed at said plane to build up charge in said fluid light modulating medium, which charge produces self-erasing deformation in the surface of said layer, each such deformation acting to diffract light directed at said layer from a light source in a light opticalsystem, the diffracted light being projected by the optical system as a function ofeach deformation to form a quick succession of images at repetition rates of greater than one image per second utilizing interlacing in at least one raster, the improvement comprising introducing into the system a fluid light modulating medium exhibiting that viscoelastic behavior characterized by the formation of a filament having a length-to-diameter ratio of at least about 100 to 1 interconnecting the free surface of such fluid medium with a pointed probe dipped into and then withdrawn therefrom.

16. The improvement recited in claim 15 wherein the fluid light modulating medium has a viscosity in the range of from about 100 centipoises to about 100,000 centipoises. 

